1. Field of the Invention The invention relates to automotive antitheft devices utilizing surface acoustic wave (SAW) devices, and more particularly to automotive antitheft SAW devices that can be easily encoded in situ at an automobile dealer's garage.
Motor vehicles are susceptible to theft due to their relative high value and inherent transportability. Automobiles and trucks are especially attractive as objects of theft, as evidenced by an alarmingly high theft rate in many civilized countries.
Recently, it has been proposed to employ surface acoustic or acoustical wave devices as security devices for automotive vehicles. See, for example, the above-cited related applications.
A SAW system generally includes an interrogator for transmitting a first radio frequency (RF) signal and a SAW transponder which receives the signal, processes it and sends back a second RF signal containing encoded information. Transducers arranged at prescribed spaced intervals along the acoustic path of the transponder convert the acoustic wave back into electrical energy. The presence or absence of transducers at prescribed locations along the acoustic wave path determines whether a reply pulse will be transmitted with a particular time delay in response to an interrogation pulse. This determines the informational code contained in the transponder reply.
A significant difficulty is presented by the logistical problems of manufacturing and distributing SAW devices when a great number of codes is possible. As one example, consider the difficulty of replacing a lost key needed for motor vehicle ignition or security system activation if the key contains a SAW transponder that must be programmed in the factory. If the possible number of embedded codes is great, then either a local distributor must stock a large number of replacement key blanks, or the user must wait a considerable time for a replacement to be shipped. Both solutions are commercially unacceptable. Many other situations involving a need to program SAW transponders locally can also be imagined.
It would be highly desirable to provide a means for encoding SAW transponders in situ, such as in the automobile dealer's garage.
It is, therefore, an object of the present invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the programmability of SAW devices.
These objects are accomplished, in one aspect of the invention, by the provision of programmable or switchable reflective transponders, together with a method of making same. The basic programmable reflector consists of a split-electrode transducer which when electrically short-circuited does not reflect surface waves, but which when open-circuited partially reflects incident surface waves. Each programmable reflector in the as-manufactured state is short circuited by a fusible link. Programming is accomplished in situ by destroying the fusible link, thereby causing the programmable reflector to be open-circuited.